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On a dyno an old tech GM 2.5 liter 4 cylinder would use 1 unit of fuel to produce 20 HP. At 50 HP the fuel consumption was 1.5 units of fuel.
Basically that means the 20 HP per unit went to 33.3 HP per unit when the load was increased. In a motorcycle with a small engine you can increase the load easily with a significant sprocket change and lower final drive ratio.
In many cases in bikes the first gear is very low to get the bike going with a max load, which in many cases is as much as the bike's weight itself, unlike a car where it is a much lower percentage of total vehicle weight.
Decreasing the total aerodynamic drag reduces the load, but it can be offset to a point by lower BSFC. The combination of lower final drive ratio, say from 3 to 1 to 2 to one, will increase the BSFC and offset the loss (in BSFC due to lower load) to improved aerodynamics. Coast down testing with some aero improvements in a bike and a good tuck position of the rider can really make a significant difference in highway speed MPG.
I once read that 90% of the energy required to get a bicycle to 30 MPH is aero drag. This is why some bike (bicycle not motorcycle) riders have hit 70 MPH when they were closely following a vehicle with a fairing on the rear of the lead vehicle to provide maximum reduction in aero drag to the bicycle.
regards
Mech
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